Recent studies by the Air Force have shown that 50% of all material failures in aircraft are a result of fatigue (1). This high incidence of failures prompted the new safe-crack-growth approach for the design of new aerospace structural systems. However, accurate calculations require a knowledge of fatigue crack growth behavior under a wide variety of load and environmental conditions. Consequently, understanding the mechanisms involved in the initiation and propagation of fatigue cracks in metals is one of the key factors in designing aircraft that are safe, efficient, and economical. Since fatigue crack initiation is a surface phenomenon and fatigue crack propagation is a bulk phenomenon, the fatigue properties may be optimized by production processes that develop the desired microstructures for FCI resistance on the surface, and the desired microstructure for FCP resistance throughout the bulk. The objective of this program is to optimize the microstructure of high strength aluminum alloys for overall fatigue resistance, i.e., resistance to both FCI and FCP, through the use of new primary processing methods. Specifically, this research will identify those microstructural features that control the different aspects of fatigue, and establish methods for incorporating those features in a finished product.